linux-kernel - context tracking vs. syscall_trace_leave & do_notify

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Date:	Thu, 30 Apr 2015 21:30:24 -0400
From:	Rik van Riel <riel@...hat.com>
To:	Fr馘駻ic Weisbecker <fweisbec@...il.com>
CC:	Andy Lutomirski <luto@...capital.net>, X86 <x86@...nel.org>,
	linux-kernel@...r.kernel.org, Thomas Gleixner <tglx@...utronix.de>,
	Denys Vlasenko <dvlasenk@...hat.com>
Subject: context tracking vs. syscall_trace_leave & do_notify_resume loop

Andy pointed out to me something I should have seen earlier: both
syscall_trace_leave and do_notify_resume call both user_exit()
and user_enter(), which has the potential to greatly increase the
cost of context tracking.

I believe (though it is hard to know for sure) there are legitimate
reasons why there is a loop around syscall_trace_leave and
do_notify_resume, but I strongly suspect the context tracking code
does not need to be in that loop.

I suspect it would be possible to stick a call to a new function
(return_to_user ?) right after the DISABLE_INTERRUPTS below, which
could be used to do the context tracking user_enter just once, and
later on also to load the user FPU context (patches I have sitting
around).

syscall_return:
        /* The IRETQ could re-enable interrupts: */
        DISABLE_INTERRUPTS(CLBR_ANY)
        TRACE_IRQS_IRETQ

Andy, Denys, do you guys see any issues with that idea?

I realize that would mean a RESTORE_EXTRA_REGS after that call
to return_to_user(), but it looks like that could be achieved
without making the code any worse than it already is :)

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